1. Technical Field
The present invention relates to battery circuits, and particularly relates to a battery charging control circuit.
2. General Background
Generally, battery chargers either use a constant current (CC) mode or a constant voltage (CV) mode to charge a battery. In the CC mode, the battery chargers charge the battery with a constant current, and in the CV mode, the battery chargers charge the battery with a constant voltage. However, in either the CC mode or the CV mode, battery charging is terminated once the battery's voltage reaches a peak value (e.g., 4.2 V), resulting in the battery having not been sufficiently charged.
Nowadays, battery chargers has been improved by charging the battery in two phases. In the first phase the battery chargers charge the battery with a large current and in the second phase, the battery chargers charge the battery with a much smaller current. The second phase follows the first phase to sufficiently charge the batteries. Referring to FIG. 3, a circuit diagram of a conventional battery charging circuit using two phases, the circuit includes a bipolar junction transistor (BJT) circuit G1. A power supply P4 is provided for supplying power to the circuit. An anode and a cathode of the P4 are respectively connected with input ports a and b of the circuit. The input port a is further connected to an emitter and a collector of the BJT circuit G1. A base of the BJT G1 is grounded via a bypass resistance RB. A battery B4 is charged when being connected between a collector of the BJT G1 and ground. Collector current of the BJT G1 is used to charge the battery B4. The collector current is generally very large when the battery B4 is empty and declines gradually when the charging continues because that the voltage of the battery B4 rises gradually. The collector current is declined to a very low level when the battery B4 approaches to a sufficient charged state.
However, BJTs are not reliable when they are used by the circuit of FIG. 3 to charge a battery in two phases. For example, BJTs tends to undergo temperature drift. This results in that the circuit of FIG. 1 can't control battery charging very precisely.
Therefore, it is necessary to provide an apparatus and method to overcomes the above-identified deficiencies.